A semiconductor magnetic electron multiplier was constructed and its characteristics were evaluated using a mass spectrometer. A detector using two multipliers with variable separation was also constructed capable of comparing two ion beams simultaneously over a wide mass range. This double detector gave ratios near the statistical limits of accuracy when two isotopes of tungsten were analysed using an ion beam with intensity varying greatly over short time periods. This electron multiplier has a wide field of application where small size and weight are an important consideration.